The present invention concerns quartz resonators and relates more particularly to resonators of the tuning fork type, which can be used in timepieces.
The relatively low frequency of a quartz fork, its good shock resistance, its mode of manufacture, and the way in which it is fitted, which can be closely conrolled, explain why it is the resonator that is currently most widely used in timepieces. The increasing demand for a higher degree of accuracy has caused manufacturers to seek to improve the levels of performance thereof, and, in particular, to improve its thermal properties. W. P. Mason, in an article entitled "A New Quartz-Crystal Plate", presented in January 1940 in "Bell Telephone System Technical Publications", showed how the thermal properties of a resonator could be influenced by coupling a number of modes of vibration. More recently, two applications of that principle to tuning fork arrangements have been disclosed, by Suwa Seikosha Co., Ltd., at the 33rd and 34th "Annual Frequency Control Symposia" in 1979 and 1980, respectively. The first presentaton, by E. Monosaki et al, entitled "New Quartz Tuning Fork With Very Low Temperature Coefficient", concerns a tuning fork which vibrates in accordance with the fundamental flexural mode and in which the first-order and second-order temperature coefficients are nullified, by virtue of coupling to the fundamental torsional mode. The second presentation, by S. Kogure et al, entitled "New Type Twin Mode Resonator" also concerns a tuning fork which vibrates in the flexural mode, but on the first harmonic, and which makes use of coupling to the fundamental torsional mode. In both cases, the coupling effect is used to modify the thermal properties of a tuning fork which vibrates in the flexural mode and in which the frequency vs. temperature curve, which is initially parabolic, is converted into a cubic curve. The major disadvantage of those prior designs is that the coupling effect envisaged considerably increases the dispersion of the first-order temperature coefficient which must therefore be the subject of an individul adjustment operation, thereby increasing the manufacturing cost of such a resonator. Moreover, the angles of cut and the vibration mode selected are such that, without the coupling effect, the first-order temperature coefficient is of a high value, the effect of the coupling influencing both the first-order coefficient and the second-order coefficient.